1. Field of the Invention
The present invention relates to a plasma oxidation method and a plasma oxidation apparatus, and more particularly to a plasma oxidation method and a plasma oxidation apparatus for forming a silicon oxide film by oxidizing silicon through radiating oxygen plasma onto a silicon substrate used in a semiconductor process.
2. Description of the Related Art
In a manufacturing process for a semiconductor apparatus, the process for forming a silicon oxide film on the surface of a silicon substrate by oxidizing the substrate, for example, is important. A method of oxidizing silicon typically employs a thermal oxidation method, but this involves a high processing temperature of 1000° C., and there is a possibility that impurities in the silicon substrate are diffused. Therefore, silicon oxidation methods using plasma oxidation has been proposed.
For example, Japanese Patent Application Publication No. 11-121448 discloses a method of manufacturing a silicon oxide film in which a positive or negative bias voltage is applied to a silicon substrate and the silicon substrate is kept at room temperature in such a manner that plasma oxidation of the surface of the silicon substrate is performed, in order to create an ultra-thin silicon oxide film which displays MOS characteristics, at room temperature.
Furthermore, Japanese Patent Application Publication No. 2005-294551 discloses an oxidation processing method of a silicon processing object, such as a silicon substrate, in which a silicon processing object is exposed to a plasma including oxygen radicals, and anisotropic oxidation is carried out by applying a DC voltage to the substrate, in order to carry out anisotropic oxidation of the silicon processing object at a lower temperature than 1000° C.
Furthermore, Japanese Patent Application Publication No. 2004-47950 discloses a method of manufacturing a semiconductor apparatus in which, in order to form a processed film thickly and uniformly, and inexpensively and at low temperature when performing nitrogen processing or oxidation processing of the surface of a substrate, an oxide film is formed by using a modified magnetron plasma processing apparatus which is capable of controlling the energy of ions injected into the substrate independently of the generation of plasma, supplying a gas including elemental oxygen into a processing chamber, and performing plasma discharge in the gas including elemental oxygen by means of a high-frequency electric field obtained by supplying high-frequency power to a cylindrical electrode and a magnetic field obtained by means of a magnetic field line forming means, so as to oxidize the processing object.
Moreover, Japanese Patent Application Publication No. 2002-280369 discloses an oxide film forming apparatus for a silicon substrate in which, in order to form a silicon oxide film which is compatible with more increased density and finer configuration of integrated circuits, a plasma including oxygen is generated and negative oxygen ions in the generated plasma are radiated onto a silicon substrate.
However, in a silicon oxidation method using plasma oxidation, there are few practical examples of low-temperature processing at 400° C. or below, and when processing has been performed at a low temperature of 400° C. or below, the oxidation rate has been slow and it has been difficult to create a thick oxide film. Furthermore, in a plasma oxidation method using positive ions and negative ions, when one of “positive ions” and “negative ions” is used, due to the occurrence of a charging-up phenomenon, the oxidation rate falls and it is difficult to oxidize a thick film.
For example, according to the method described in Japanese Patent Application Publication No. 11-121448, when oxidizing very thin films such as a semiconductor, the oxidation rate is low, and therefore it is possible to control the film thickness and productivity is not a problem. However, if a positive bias is applied to a silicon substrate, negative ions arrive mainly at the substrate, and if a negative bias is applied, positive ions arrive at the substrate. In this way, according to a method which applies one of a positive bias and a negative bias, in other words, a method applying a DC (direct current) bias, a charging-up phenomenon occurs and it is difficult to form a thick oxide film, and furthermore, there is a problem of poor productivity due to the low oxidation rate.
Moreover, in the method described in Japanese Patent Application Publication No. 2005-294551, anisotropic oxidation is achieved by using ions in such a manner that the upper surface and bottom surface of a projecting shape are oxidized but the side faces are not oxidized. However, if a DC voltage is applied to a substrate, there is a possibility that a charging-up phenomenon may occur due to attraction of one of the positive ions and negative ions. Moreover, due to effects of the charging-up phenomenon, as well as being difficult to form a thick oxide film, the oxidation rate is low and productivity is poor.
Furthermore, according to the method described in Japanese Patent Application Publication No. 2004-47950, the potential of the substrate is controlled by a variable impedance, and oxidation is carried out by controlling the energy of the ions. However, if the substrate potential is controlled by a variable impedance, then it is not possible to control the value of the self-bias voltage Vdc, to a positive potential, and furthermore, the value of Vdc changes simultaneously with the control of the peak-to-peak voltage Vpp. Therefore, since Vdc and Vpp cannot be controlled individually, there are limitations on the range of the oxidation conditions, and consequently, it is difficult to achieve a thick oxide film and to improve the oxidation rate.
Furthermore, according to the method described in Japanese Patent Application Publication No. 2002-280369, oxidation is carried out by irradiating negative ions onto the substrate, and the negative ions are radiated by applying RF (high-frequency wave) and DC (direct current) by transformer-coupling. However, if only negative ions are used, then there are possibilities that it is difficult to achieve a thick film due to the occurrence of a charging-up phenomenon, and the oxidation rate is low. Furthermore, if the peak-to-peak voltage Vpp during oxidation and the self-bias voltage Vdc are too high, then plasma forms in the vicinity of the stage and it is difficult to achieve oxidation which causes little damage. Moreover, in Japanese Patent Application Publication No. 2002-280369, bias application frequencies of 400 kHz and 1 MHz are used, and in the case of the lower frequency, the oxide film becomes a capacitor and it becomes difficult to apply the bias. Therefore, when forming a thick oxide film, it is difficult to obtain the effects of applying the bias, and therefore it is difficult to obtain a thick film.